In a flat-type display device, there has been wide use of an active matrix type display device, in which a self luminous element such as an OLED (Organic Light Emission Diode), or a liquid crystal element is used as an optical modulation element, and a TFT (Thin Film Transistor) gate for addressing is provided on each pixel.
Here, in the active matrix type display device, a plurality of data lines and a plurality of select lines which cross respective data lines at right angles, are provided, and pixels are provided on respective crossing points of the data lines and the select lines. When a case of using the OLED as the optical modulation element is used as an example, as shown in FIG. 18, a select module 113 is conducted only in a select period in which a select line 103 is outputting a select signal SEL of a select level, and the select module 113 connects a data line 102 to a drive module 111 which drives the OLED 112.
While, in the drive module 111, a TFT 121 is provided between a power line Lr, to which a reference potential Vref is applied, and the OLED 112. A capacitor 122, which functions as a memory element, is connected to a gate of the TFT 121, and stores a data signal DATA in a select period, and the data signal DATA is applied to the gate of the TFT 121 also in a non-select period. Note that, like a pixel 104a shown in FIG. 19, the OLED 112 may be provided between the TFT 121 and the power line Lr.
However, in each of the pixels 104 (104a), since the data signal DATA is stored as the analog quantity, as shown in FIG. 20, a signal level of the data signal DATA, applied in the select period, declines gradually in the non-select period, due to a leak current in a circuit, and the like.
Thus, it is required that select periods are set cyclically, and a time changing rate of a potential, stored by the capacitor 122, is adjusted to such extent that the potential declining quantity in the cycle of the select period does not influence the display, for example, by setting a capacitance of the capacitor 122, and the like. Further, the capacitance, required by the capacitor 122, is determined in accordance with a display gradation number, but a capacitance, which can be formed in each (104a) of the pixels 104, is restricted, so that a gradation number, which can be displayed, or a cycle of the select periods is restricted.
Thus, Japanese Unexamined Patent Publication No. 161564/1998 (Tokukaihei 10-161564) (publication date: Jun. 19, 1998) proposes a display device, having a structure in which a voltage drive type EL element is used as an optical modulation element, wherein a gate insulating film of the TFT 121 is formed by using a nitriding silicon film in which an impurity ion is doped, so as to give an EEPROM function to the TFT 121 instead of providing the capacitor 122. Further, Patent Gazette No. 2775040 (registration date: May 1, 1998) discloses an optical modulation element, having a structure in which a voltage drive type liquid crystal is used, wherein a ferroelectric capacitor stores a data signal DATA. According to the structures, unlike the structures shown in FIG. 18 and FIG. 19, a decline of a potential level is restricted, so that it is possible to store the data signal DATA for a long time.
Further, as another structure which is different from the structure in which the data signal DATA is stored as the foregoing analog quantity, for example, Japanese Unexamined Patent Publication No. 194205/1996 (Tokukaihei 8-194205) (publication date: Jul. 30, 1996) and Japanese Unexamined Patent Publication No. 119698/1999 (Tokukaihei 11-119698)(publication date: Apr. 30, 1999) propose a structure in which, like the pixel 104b shown in FIG. 21, a memory element 123, provided instead of the capacitor 122, stores a binary of light/light-off of an optical modulation element, and a gradation display is performed in accordance with an area modulation. According to the structure, since the binary is stored, it is possible to store the data signal DATA for a long time, compared with a case of storing as the analog quantity.